Parity-violating DIS and the flavour dependence of the EMC effect

Isospin-dependent nuclear forces play a fundamental role in nuclear structure. In relativistic models of nuclear structure constructed at the quark level these isovector nuclear forces affect the u and d quarks differently, leading to non-trivial flavour dependent modifications of the nuclear parton distributions. We explore the effect of isospin dependent forces for parity-violating deep inelastic scattering on nuclear targets and demonstrate that the cross-sections for nuclei with N /= Z are sensitive to the flavour dependence of the EMC effect. Indeed, for nuclei like lead and gold we find that these flavour dependent effects are large.Comment: 4 pages, 2 figure

Pion distribution amplitude from lattice-QCD

A method is explained through which a pointwise accurate approximation to the pion's valence-quark distribution amplitude (PDA) may be obtained from a limited number of moments. In connection with the single nontrivial moment accessible in contemporary simulations of lattice-regularised quantum chromodynamics (QCD), the method yields a PDA that is a broad concave function whose pointwise form agrees with that predicted by Dyson-Schwinger equation analyses of the pion. Under leading-order evolution, the PDA remains broad to energy scales in excess of 100 GeV, a feature which signals persistence of the influence of dynamical chiral symmetry breaking. Consequently, the asymptotic distribution, \phi_\pi^asy(x), is a poor approximation to the pion's PDA at all such scales that are either currently accessible or foreseeable in experiments on pion elastic and transition form factors. Thus, related expectations based on \phi_\pi^asy(x) should be revised.Comment: 5 pages, 2 figure

The NJL-jet model for quark fragmentation functions

A description of fragmentation functions which satisfy the momentum and isospin sum rules is presented in an effective quark theory. Concentrating on the pion fragmentation function, we first explain why the elementary (lowest order) fragmentation process q --> q \pi is completely inadequate to describe the empirical data, although the "crossed" process \pi --> q \bar{q} describes the quark distribution functions in the pion reasonably well. Taking into account cascade-like processes in a generalized jet-model approach, we then show that the momentum and isospin sum rules can be satisfied naturally, without the introduction of ad hoc parameters. We present results for the Nambu--Jona-Lasinio (NJL) model in the invariant mass regularization scheme and compare them with the empirical parametrizations. We argue that the NJL-jet model, developed herein, provides a useful framework with which to calculate the fragmentation functions in an effective chiral quark theory.Comment: 21 pages, 7 figure

Isovector EMC effect explains the NuTeV anomaly

A neutron or proton excess in nuclei leads to an isovector-vector mean-field which, through its coupling to the quarks in a bound nucleon, implies a shift in the quark distributions with respect to the Bjorken scaling variable. We show that this result leads to an additional correction to the NuTeV measurement of sin^2(Theta_W). The sign of this correction is largely model independent and acts to reduce their result. Explicit calculation within a covariant and confining Nambu--Jona-Lasinio model predicts that this vector field correction accounts for approximately two-thirds of the NuTeV anomaly. We are therefore led to offer a new interpretation of the NuTeV measurement, namely, that it is further evidence for the medium modification of the bound nucleon wavefunction.Comment: 4 pages, 2 figure

Commentary on rainbow-ladder truncation for excited states and exotics

Ground-state, radially-excited and exotic scalar-, vector- and flavoured-pseudoscalar-mesons are studied in rainbow-ladder truncation using an interaction kernel that is consonant with modern DSE- and lattice-QCD results. The inability of this truncation to provide realistic predictions for the masses of excited- and exotic-states is confirmed and explained. On the other hand, its application does provide information that is potentially useful in proceeding beyond this leading-order truncation, e.g.: assisting with development of projection techniques that ease the computation of excited state properties; placing qualitative constraints on the long-range behaviour of the interaction kernel; and highlighting and illustrating some features of hadron observables that do not depend on details of the dynamics.Comment: 10 pages, 6 figures, 2 table

Spin dependent parton distributions and structure functions

Nuclear parton distributions and structure functions are determined in an effective chiral quark theory. We also discuss an extension of our model to fragmentation functions.Comment: To appear in the proceedings of the 20th European Conference on Few-Body Problems in Physics, Pisa, September 10-14, 200

On unifying the description of meson and baryon properties

A Poincare' covariant Faddeev equation is presented, which enables the simultaneous prediction of meson and baryon observables using the leading-order in a truncation of the Dyson-Schwinger equations that can systematically be improved. The solution describes a nucleon's dressed-quark core. The evolution of the nucleon mass with current-quark mass is discussed. A nucleon-photon current, which can produce nucleon form factors with realistic Q^2-evolution, is described. Axial-vector diquark correlations lead to a neutron Dirac form factor that is negative, with r_1^{nu}>r_1^{nd}. The proton electric-magnetic form factor ratio falls with increasing Q^2.Comment: 5 pages, 4 figures, 1 tabl

Chiral susceptibility and the scalar Ward identity

The chiral susceptibility is given by the scalar vacuum polarisation at zero total momentum. This follows directly from the expression for the vacuum quark condensate so long as a nonperturbative symmetry preserving truncation scheme is employed. For QCD in-vacuum the susceptibility can rigorously be defined via a Pauli-Villars regularisation procedure. Owing to the scalar Ward identity, irrespective of the form or Ansatz for the kernel of the gap equation, the consistent scalar vertex at zero total momentum can automatically be obtained and hence the consistent susceptibility. This enables calculation of the chiral susceptibility for markedly different vertex Ansaetze. For the two cases considered, the results were consistent and the minor quantitative differences easily understood. The susceptibility can be used to demarcate the domain of coupling strength within a theory upon which chiral symmetry is dynamically broken. Degenerate massless scalar and pseudoscalar bound-states appear at the critical coupling for dynamical chiral symmetry breaking.Comment: 9 pages, 5 figures, 1 tabl